TO DIVE O R... Tony Foale January 1997

Transcription

1 TO DIVE O R... Tony Foale January 1997 Over the past couple of years the subject of anti-dive has received much attention, but there seems to be some divergence of opinion over the value of the Japanese solution to the problem i.e. the system where-by the compression damping force in the front forks is increased by the application of the front brake, thus slowing the rate of dive, not eliminating it. Avid road test readers will no doubt have noticed the favourable comments made recently by some testers after riding European machines not fitted with such a device. It doesn't take much imagination to see that comfort and road holding may suffer on bumpy roads with the stiffened up damping, and it is interesting to look at one of the latest developments from Yamaha on their moto-cross machines, BASS or "brake actuated suspension system", for the uninitiated, this is a system that REDUCES the bump damping when the brake is applied in order to lessen wheel hop over bumpy ground. So, is anti-dive desirable or is it just another "hi-tech" gimmick, if it is useful is there perhaps a better way? To answer these questions and more, we should first consider why normal forks (telescopies) dive so much in the first place. TELESCOPICS. There are two sources of dive associated with teles, --one is the obvious effect of weight transfer, which is dependent on the C of G. height and the wheelbase, the other is a less obvious effect due to the rearward rake of the fork legs. This rake means that the braking force on the front tyre is split into two components when fed into the forks, one is in line with the sliders and hence tends to compress the springs ( this force is approximately 50% of the braking force.), the second component is at right angles to the forks and this tries to bend the fork legs ( roughly 80% of the braking force ). On a typical super-bike the increased force in the fork springs due to weight transfer is around 40% of the braking force, so we see that the dive tendency is more than doubled by the addition effect of the angled sliders. In the absence of any anti-dive system ( remember that the current hydraulic systems only slow the rate of dive, they do not eliminate i t ), there are two ways to accommodate this effect, -use stiff fork springs to limit the movement or use soft springs with large movement. But surely these parameters should be selected on roadholding, handling or comfort grounds. The disadvantages of hard suspension are self evident, and the large fork movement associated with today's softer springs, allows undue pitch changes and variations in steering geometry, to the detriment of comfort and stability. Fortunately there is an effective anti-dive system which the mechanically adept can fit to their own bikes, but don't expect the major manufacturers to fit it as original equipement, the design is visually messy, increases wheel changing effort and does not have the aura of "high-tech" gimmickry. Garelli currently feature it on their G.P. racers and so did Kawasaki a few years back. The brake caliper is fitted to a floating bracket which pivots on a bush or bearing around the wheel spindle, the free end of this bracket is prevented from rotating by a pivoted rod usually fixed at it's upper end to the bottom triple clamp. When the brake is applied the caliper bracket tries to rotate in the same direction as the wheel, but this is prevented by the rod which in turn pushes up under the fork yolk, thus acting in opposition to the diving tendency. The length of the caliper bracket controls the degree of anti-dive, the longer the bracket the smaller will be the anti-dive effect. Indeed if the bracket is too short, then we may have the opposite problem, -the front rising under braking. Anyway what's all this talk about telescopic forks? There are better alternative ways of providing motorcycle front suspension/steering. ALTERNATIVES TO TELESCOPIC. According to many press reports these alternatives usually have what is frequently called "natural anti-dive". I am not sure just what that means but if the Japs, ever start to use these schemes then their over-worked acronym departments will have a field day,nads, to start with. -B ut let's forget the jargon and just consider the characteristics ofthese designs. Firstly, let me introduce and explain the concept of "percentage anti-dive". Let's define 100% anti-dive as that which allows neither dive nor rise under the action of the front brake only (Seems logical). Also define 0% anti-dive as that which permits an amount of dive equal to that due only to the effect of weight transfer. We have already seen that telescopic forks cause a greater dive than this, and so we can represent them with a negative anti-dive percentage, i.e. the opposite to anti-dive. Now, what follows is a simple method of finding the anti-dive percentage of most suspension systems. While the method fi I e :///F :/M otorcycle/stuff% 20m oved%20firom % 20l egacy% 20dri ves/tfoale/d IVE. H TM 1/9

2 requires only a pencil and a ruler the reasoning behind it is a little bit more inclined to cause a sore head, space prevents a full exploration of the theory sufficient to satisfy the purists but I will attempt to make an abridged version comprehensible and convincing. Consider the Elf.E. design, if we allow a very small wheel displacement to take place then the line of movement of the forward ends of the swing arms will be at right angles to those arms. Therefore the length of the arms does not affect the direction of motion of the upright ( for small wheel movements). Imagine for the moment that these arms converge towards the rear of the machine, draw lines though through them until the lines meet. Consider now, that both swing arms are pivoted at this meeting point, as we have seen the motion of the upright will be unaffected. Also as the two arms now pivot around the same point we could replace them with a single arm firmly affixed to the upright. Because this new single swing arm is only a figment of our imagination, let's call it a "virtual swing arm" and its pivot a "virtual pivot". Now, for small movements of the wheel we can replace the two real arms with our single "virtual arm", for the purpose of analysing the dive characteristics. Neglecting the forces in the suspension unit, for the time being, ( it is only the residual dive forces that change those ), we can see from Fig.l that the braking forces at the tyre can only be fed into the bulk of the bike along the "virtual arm" to the "virtual pivot". This force acting through the pivot is reacted against the inertia of the machine acting through the centre of gravity ( C of G ). If you are still with me after that, we are now ready to look at some examples and see how it works. Fig.2. shows a layout which places the "virtual pivot" at ground level, therefore the braking force is passed into the bike at this level in a horizontal direction but this is reacted through the C of G. and gives rise to a moment equal to the braking force multiplied by the C of G. height. This couple is resisted by an increased load on the front wheel and a decreased load on the rear. -i.e. weight fi I e :///F :/M otorcycle/stuff% 20m oved%20firom % 20l egacy% 20dri ves/tfoale/d IVE. H TM 2 /9

3 transfer. So in this case the dive tendency is due only to weight transfer, or in other words, 0% anti-dive. Consider now the example in Fig.3. In this case the virtual pivot lies on a line drawn through the tyre contact patch and the C of G. Weight transfer cannot be prevented by an internal rearrangement of the swing arms, but the vertical component of the force acting through the virtual pivot ( and also through the tyre contact patch) balances any attempt to dive, hence we have 100% anti dive. Fig.4. shows another example, but here the line through the contact patch and the virtual pivot passes halfway up the vertical line through the C of G. this gives 50% anti-dive. Yes, now you have it, the percentage of the vertical line through the C of G. left under the line through the virtual pivot gives us the anti-dive percentage In the initial explanation I made a point of considering only small wheel movements, this is because with larger displacements the changed angles of the actual swing arms result in a new position for the "virtual pivot" and hence the antidive percentage may change through out the range of wheel travel. The change possible with parallel equal length arms is illustrated by Fig.5., when the suspension is unloaded the anti-dive is at a maximum, and at full compression we change to negative anti-dive after passing through 0% in the mid position. fi I e :///F :/M otorcycle/stuff% 20m oved%20firom % 20l egacy% 20dri ves/tfoale/d IVE. H TM 3 /9

4 Let's examine this in more detail. Suppose that when braking on a smooth road the anti-dive is sufficient to allow only a small compression of the suspension, now, if we hit a sizable bump the wheel will rise but we know that this will also reduce the anti-dive, and so the suspension will be further compressed and dive enhanced. Thus under braking a bump will result in a greater wheel deflection than if the same bump were hit at the same speed, without braking. This means that the effective spring rate is reduced as the brakes are applied, the more brakes the softer will be the rate. One often sees comments to the effect that parallelogram designs remove all braking effects from the suspension, we can see that this is clearly not tme. -In order to keep a constant effective spring rate (assuming that the suspension is not progressive, anyway) we need to have a constant anti-dive percentage throughout the full range of wheel travel, although unless the percentage is 100. there will still be some initial compression depending on the braking severity. It is possible to design a double link system to give a constant anti-dive with the percentage of our choice, and we can use the simple line drawing methods that we have used to analyse existing schemes. Say we want a system which gives 50% over the full range of suspension movement, we need a starting point, so at one extreme of suspension travel, let's draw in the lower swing-arm in any position that seems about right ( perhaps dictated by a suitable mounting lug ), now draw in the line through the tyre contact-patch which corresponds with 50%, also draw in a line extending the lower swing-arm, as in Fig.6. the meeting point of these lines is the "virtual pivot" location required for that particular suspension position. If we now draw fi I e:///f :/M otorcycle/stuff%20m oved%20firom %20l egacy%20dri ves/tfoale/d IVE. H TM 4/9

5 in a line connecting the virtual pivot and the upper joint on the upright, this now defines the line along which the upper arm lies. So the only thing we now need is to establish the length of this arm. To do this we must redraw the wheel and suspension at the other end of its travel, and repeat the above procedure. There are now two intersecting lines which describe the upper arm location, and hence the point of intersection is the required pivot point for that arm. The above is only an approximation since we have not taken into account any effects from the rear end of the bike, there are two complications, one is the additional weight transfer due to any rear braking and the other is the rise (opposite to dive) of the back end. This rise is dependent on weight transfer and also on the layout and spring rate of the rear suspension. However, even totally ignoring these effects we still can get a good idea of the behaviour of the front end. The Elf.E. system was used to illustrate the principle of how we can determine the dive characteristics of a motorcycle, but any design for which we can define a "virtual swing-arm" can be analysed in like manner. This includes all of the hub-centre and double-link systems, leading and trailing link forks and even teles. If telescofpic fork legs are replaced by an infinitely long virtual swing-arm mounted at the wheel spindle and at right angles to the sliders, then the wheel motion will be unchanged and the line through the contact patch and the virtual pivot will again define the anti-dive percentage, Fig.7. shows that we get a negative value which agrees with the previous reasoning. fi I e :///F :/M otorcycle/stuff% 20m oved%20firom % 20l egacy% 20dri ves/tfoale/d IVE. H TM 5 /9

6 There is a certain school of thought which reasons that dive is desirable because it reduces the C of G. height and weight transfer along with it, hence allowing the rear brake to take on a bigger share of the stopping operation. But, this forgets that if dive is eliminated or reduced then suspension travel can be reduced also, this in turn means that we can lower the whole bike before running into cornering clearance problems, i.e. the C of G. can be lower anyway. REAR END The rear end suffers from the opposite problem, --rise not dive --this lifts the C of G. as well as increasing the pitch change, and thus is undesirable. Perhaps we can use the reactions from the rear brake to reduce the effect? In fact we can, and we can also use similar methods to analyse the behaviour. Look at a normal rear swinging arm with the caliper fixed to it, fi I e :///F :/M otorcycle/stuff% 20m oved%20from % 20l egacy% 20dri ves/tfoal e/d IVE. H TM 6 /9

7 Fig.8. shows how the anti-rise depends on the swing arm length ( in this case we can use the actual swing-arm, there is no need for a virtual one ), a short arm may give more than 100% anti-rise, i. e. the rear may actually sink down under the action of the rear brake only. The rising tendency, caused by the weight transfer from front wheel braking, calls for more drastic solutions. fi I e :///F :/M otorcycle/stuff% 20m oved%20from % 20l egacy% 20dri ves/tfoal e/d IVE. H TM 7 /9

8 The layout in Fig. 9. shows a floating brake mounting but with the torque stay mounted high up and well back, the virtual pivot for this arrangement is at the intersection of the swing-arm and the torque stay, we can see that this gives a very high anti-rise percentage. On the other hand the normal floating brake designs (with perhaps a parallelogram linkage) give a very low anti-rise effect. So which is best? It depends on the application, the large anti-rise design can give problems with juddering and wheel hop. As the brake is initially applied the anti-rise forces tend to draw the rear wheel and rest of the bike together ( i.e. compress the suspension), but because the inertia of the sprung part of the bike is greater than that of the unsprung rear there is a tendency for the wheel to rise off the ground before the rest of the bike comes down to meet it. Even if the wheel does not actually leave the ground, the load on it is momentarily relieved and may easily lock up, the sequence of events may repeat and wheel hopping is then under way. Riding style can determine whether this becomes a problem, a rider who applies the brake in a gentle fashion may never experience trouble whereas someone with a less delicate touch may find it totally uncontrollable. Bumpy surfaces will obviously aggravate the effect. That is why moto-cross machines use the more normal (parallelogram or similar) floating brake design and road bikes so equipped are usually nicer to brake with than those with the caliper fixed to the swing-arm. Anyway the problem of rise on the rear is less severe than that of dive on the front, the springing is stiffer on the rear and so for a given weight transfer the rise will be less than the dive, particularly with the additional dive effect due to the raked fork legs anyway. How much anti-dive do we need anyway? This depends on rider preference, but my experience has been that no dive at all causes a lack of feel for the braking process, but about 50% anti-dive gives sufficent control with a minimum of dive(around 20-25% of the dive from teles). To conclude. We have seen that telescopic forks have a fundamental dive problem which is not fully solved by the current popular methods. Other suspension designs, such as the various "hub-centre" systems and leading/trailing link forks offer the potential for building in any desired degree of anti-dive, but a true parallelogram gives rise to undesirable changes throughout the full range of wheel travel. Designs using non-parallel and unequal length links can offer nearly constant properties over the whole suspension movement. I didn't mention it before but these constant dive characteristics are bought at the expense of a changing steering geometry, whether this is good or bad will have to wait for another time. fi I e :///F :/M otorcycle/stuff% 20m oved%20firom % 20l egacy% 20dri ves/tfoale/d IVE. H TM 8/9